Solar receiver with a plurality of working fluid inlets

ABSTRACT

A solar receiver ( 2 ) comprises a housing ( 4 ) defining a receiver chamber ( 12 ) and an aperture ( 14 ); a window ( 16 ) mounted in the aperture ( 14 ); at least two inlet means ( 17 ) axially spaced from the window ( 16 ) and positioned at different distances therefrom for the injection into the receiver chamber ( 12 ) of different flows of workind fluid; an outlet means ( 18 ) for the ejection of the working fluid out of the receiver chamber ( 12 ); and absorption control means for the provision of the different flows of the working fluid with different capability to absorb solar radiation

FIELD OF THE INVENTION

The invention relates to a solar receiver designed for admittingconcentrated solar radiation and converting its energy into another formof energy.

BACKGROUND OF THE INVENTION

A receiver of the kind to which the present invention refers typicallyhas a housing with an interior space called a receiver chamber, and awindow mounted in the housing to allow the concentrated radiation toenter the receiver chamber. The receiver housing also includes an inletfor the ingress into the receiver chamber of a working fluid which, whenheated by the concentrated radiation, converts the heat into anotherform of energy. The working fluid may comprise components which, whenheated, perform a chemical reaction. The receiver housing also comprisesan outlet for the egress of the working fluid, possibly with thereaction products, from the receiver chamber. Examples of a solarreceiver of this kind may be found in U.S. Pat. No. 4,313,304 and WO01/12314.

In a solar receiver as described above, it is desired that all theenergy of concentrated solar radiation entering the receiver chamber beabsorbed therein by the working fluid to allow the most efficient energyconversion. However, in practice, a part of the incoming radiationabsorbed in the receiver chamber is re-irradiated back to thesurrounding through the receiver aperture. The higher the temperature ofthe chamber walls, the more radiation is emitted thereby, increasingenergy losses and thus reducing the receiver efficiency.

U.S. Pat. No. 4,499,893 attempts to solve the above problem by ensuringthat the working fluid together with chemical reactants comprisedtherein reach their maximal temperature and are withdrawn from thereceiver chamber in the closest vicinity of the window. In the receiverof U.S. Pat No. 4,499,893 the window is mounted in an aperture formed ina front wall of the receiver chamber, inlet ports are arranged in sidewalls of the chamber for the ingress of the working fluid therein atlocations remote from the window, and an outlet is in the form of anoutlet opening in a rear wall of the chamber with a quartz exhaust pipeextending from the vicinity of the window through the outlet opening tothe outside of the chamber. In consequence of this design, the workingfluid is intended to absorb most of the radiation passing through thewindow in its vicinity just before the working fluid enters the exhausttube so as to ensure that chemical reaction between the reactants of theworking fluid take place inside the tube. Thereby, heating of the wallsof the receiver is essentially reduced.

SUMMARY OF THE INVENTION

The present invention provides a solar receiver having a receiverchamber with a working fluid and a window at its front end, whereinmeasures are taken to ensure that the working fluid and, consequently,walls of the receiver chamber are heated to their highest temperaturesat a region in the receiver chamber, which is relatively remote from thewindow and, preferably, has a relatively small cross-sectional area,whereas most of the chamber has temperature that is significantly lowerand therefore re-radiation losses from the walls are substantiallyreduced.

In accordance with the present invention, this is obtained by:

-   -   the provision in the receiver chamber of at least two inlet        means spaced from said window and positioned at different        distances therefrom for the injection into the receiver chamber        of a working fluid characterized by a capability to absorb solar        radiation; and    -   the injection in said at least two inlet means of corresponding        at least two flows of the working fluid such that the capability        to absorb solar radiation of the flow of the working fluid        injected via the inlet means located farther from the window is        higher than that of the flow of the working fluid injected via        the inlet means located closer to the window.

Consequently, heating of the working fluid to the highest temperaturesoccurs farther from the window, whereby re-radiation losses through thewindow are essentially minimized.

The meaning of the term ‘spaced from the window’ used in the presentapplication and claims with respect to the at least two inlet means isthat the distance between said inlet means and the window is greaterthan that at which cooling or protecting fluid is normally directedalong the window's surface to cool or protect the window, as forexample, in WO 96/25633 or WO 01/12314.

The difference in absorption capability between the two or more flows ofthe working fluid entering the receiver chamber in accordance with thepresent invention may be obtained, for example, by the provision thereinof different concentrations of particles which absorb solar radiationand get heated thereby to efficiently heat other components of theworking fluid. In particular, in accordance with the present invention,the concentration of such particles in the working fluid flow enteringthe receiver chamber farther from the window is higher than in thatentering the receiver chamber closer thereto.

In the receiver of the present invention, the concentration of particlesin the different flows of the working fluid as well as the kind of suchparticles may be controlled automatically depending on workingconditions.

In addition to the control of the concentration of the solar absorbingparticles, it may be useful to have independent control of suchparameters of the working fluid flows entering the receiver chamber astheir rates and angles at which the flows are injected.

The effect of lowering energy losses by means of the invention isincreased dramatically as the required overall operating temperature ofthe receiver increases. Without being bound to theory, this may beexplained by the known dependency of the re-radiation losses on thefourth power of the temperature:Reradiation losses˜∫εT ⁴ F dA,

wherein ε is the local emissivity of a wall area element dA, T is thelocal temperature of said area element, F is the view factor betweensaid area element and the receiver aperture, and the integral is overthe entire surface area of the chamber wall.

The exact values of the capability to absorb energy of the working fluidat its various entries to the receiver chamber may vary according to theshape of the receiver chamber, the working temperature and other workingconditions of the receiver and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a specific embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic illustration of a solar energy receiver accordingto one embodiment of the invention; and

FIG. 2 is a cross-section in the solar energy receiver of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows a solar receiver 2 according to one embodiment of thepresent invention, adapted for the admission of solar radiation highlyconcentrated (up to several thousand suns) by a suitable solarconcentrator (not shown), known in the art per se, for heating a workingfluid (not shown) in the receiver, adapted for the operation underelevated pressures of up to about 20 atmospheres. The working fluid,when heated is used for the conversion of its heat into another form ofenergy. This conversion may be performed in the receiver cavity, whenheat absorbed by the working fluid initiates a chemical reaction betweendifferent reactants comprised in the working fluid. Alternatively, theworking fluid after having been heated may be transferred to anothersite for the withdrawal of heat therefrom.

The receiver 2 includes a thermally isolated metal housing 4 with areceiver chamber 12 having a front wall 6, a side wall 7, a rear wall 8and a longitudinal central axis A extending between the front and rearwalls. The chamber 12 has a front region defined by the front wall 6 andthe adjacent front half of the side wall 7, and a rear region defined bythe rear wall 8 and the adjacent rear half of the side wall 7. As seen,the cross-sectional area of the rear region is smaller than that of thefront region.

The front wall 6 is formed with an aperture 14 holding a window 16 forthe admission of solar radiation and passing it into the receiverchamber. The aperture 14 is located in, or in the vicinity of, the focalplane of the solar concentrator. The window 16 may have any shape knownin the art per se. It could be flat, if the pressure within the chamberwere atmospheric or close thereto, but since this is normally not thecase and the pressure within the chamber is higher, the window ispreferably concave. It may also be shaped as an axi-symmetric dome, toimprove its capability to withstand high temperatures such as about 500°C. and higher. The window may also be frusto-conical and cappedfrusto-conical, or may have any other appropriate shape.

The solar receiver 2 has a plurality of inlet means 17 formed in thehousing 4 for the injection of different flows of the working fluid intothe receiver chamber 12 through inlet ports 17 a, 17 b, 17 c and 17 dformed in the side wall 7 of the receiver chamber 12. The inlet ports 17a to 17 d are all spaced from the aperture 14 to different axialdistances d_(a) to d_(d) therefrom. The rear wall 8 of the receiverchamber is formed with an outlet port 18 for the withdrawal of theworking fluid from the receiver chamber 12.

The receiver further comprises means (not shown) for the provision ofthe different flows of the working fluid injecting into the receiverchamber 14 via the inlet ports 17 a to 17 d, with different absorptioncapability, particularly, with different concentration of solarabsorbing particles. Such concentration control means are adapted tomake sure that the concentration in the working fluid flows increasesfrom its lowest value in the working fluid flow injected via the inletport 17 a to the highest value in the working fluid flow injected viathe inlet port 17 d, so that the absorption capability of the workingfluid gradually increases with the increase of the axial distance fromthe window of the location of its injection. The concentration controlmeans with which the inlet means 17 are connected may be in the form ofdifferent sources of working fluid having different parameters.Alternatively, the receiver may be associated with single source ofworking fluid free of solar absorbing particles or having some initialsmall concentration thereof, which single source is provided with a flowdistributor for dividing the working fluid into a plurality of flows tobe forwarded to different inlet means 17, and with a source of solarabsorbing particles, wherefrom particles are added to the differentflows in different amounts, in accordance with desired values ofconcentration to be obtained in these flows.

It should be mentioned that with the working fluid comprising solarabsorbing particles, the inlet port that is closest to the aperture 14should still be spaced therefrom so as to minimize contact with thewindow 16 of the solar absorbing particles comprised in the workingfluid flow injected therethrough.

It should be also mentioned that the receiver may further comprise anadditional inlet means e.g. such as disclosed in WO 96/25633 or WO01/12314 for the introduction along the window of a cooling orprotecting flow of fluid. Clearly, this flow should be completely freeof any solar absorbing particles.

The inlet ports 17 a to 17 d may further be provided with individualmeans for the independent control of other parameters of the workingfluid flow therethrough, such as for example, the flow rate and theangle at which the working fluid is injected in the receiver chamber.

FIG. 2 illustrates one example of the design of the inlet means 17 andit shows a cross-sectional view of the receiver taken through one of theinlet means, i.e. that having inlet ports 17 a. As seen, the inlet means17 includes a pipe 30 connectable to a source of the working fluid (notshown) providing a corresponding predetermined concentration of solarabsorbing particles, a manifold 31 connected to the pipe 30 and aplurality of inlet nozzles 32 terminating at a plurality of the inletports 17 a, all in fluid communication with the manifold 31. The inletnozzles 32 have all different circumferential location and asubstantially tangential orientation relative to the receiver chamber12. An angle 34 of the orientation of the inlet nozzles 32 may beindependently controllable for each of the inlet means 17.

The receiver chamber 12 is preferably elongated and converging towardsthe outlet port 18, however, generally, it may have any appropriateshape known in the art per se. It may be shaped as a cylinder, or cone,or a combination thereof, or it may be in the form of a dome, may havespindled or oval shape, or the like.

In operation, solar radiation enters the solar energy receiver 2 throughthe window 16. The working fluid is injected into the receiver throughthe inlet means 17 in a plurality of flows while the concentration ofthe solar absorbing particles in different flows, and, optionally, alsothe rate of these flows and the injection angle 34, are controlled ateach of these means to achieve a maximal temperature of the workingfluid at the rear region of the chamber. Consequently, the walls of thereceiver chamber 12 have maximal temperature and maximal re-radiation atthe rear region of the receiver chamber where the amount of re-radiationcapable to escape through the window and, consequently, the heat lossesin the receiver are minimal. Thereby, the energy-conversion efficiencyof the receiver 2 may be essentially increased in comparison to theefficiency of a similar receiver having only one inlet means or havingmore than one inlet means, but all of them adapted to inject in areceiver chamber working fluid having the same solar absorptionparameters.

EXPERIMENTAL RESULTS

Table 1 summarizes the test results from a few representative tests outof about 30, conducted separately with four different working gases in areceiver chamber of the kind to which the present invention refers. Bothwall and gas temperatures were lowest near the aperture and increasedwith distance from it. Maximum wall and gas temperatures were measurednear the gas outlet port. Table 1 shows only the temperature near thegas outlet port. As can be seen from the table, the measuredtemperatures were, in general, very high relative to those normally usedin other solar receivers. In all cases, the exit gas temperature washigher than the temperature of the chamber rear wall adjacent the gasoutlet port, demonstrating the non-isothermal effect described above.

TABLE 1 Selected test results Particle Loading Wall temperature Exit gasDT at Tg max Gas near gas outlet [K] temperature [K] [K] [g/m3] Ar 17201834 114 7.0 N2 1847 2079 232 3.8 N2 1748 2118 370 2.7 N2 1707 2008 3013.5 N2 1836 1993 157 6.0 N2 1867 2017 150 5.3 N2 1792 1962 170 5.1 N21579 1887 308 2.4 N2 1651 1843 192 5.0 Air 1725 1903 178 4.5 CO2 17481878 130 4.7 CO2 1698 1789 91 5.3 CO2 1698 1744 46 2.1 CO2 1607 1659 526.2

Although a description of specific embodiments have been presented, itwould be clear for a skilled person that variations could be madethereto without deviating from the major idea of the invention.

1. A solar receiver for receiving solar radiation and converting itsenergy into another form of energy, the receiver comprising: (a) ahousing defining a receiver chamber having a longitudinal central axisand an aperture; (b) a window mounted in said aperature and adapted forthe admission of concentrated solar radiation and passing it into saidreceiver chamber; (c) at least two inlet means axially spaced from saidwindow and positioned at different distances therefrom for the injectioninto the receiver chamber of different flows of working fluid; (d) anoutlet means for the ejection of the working fluid out of the receiverchamber; and (e) absorption control means for the provision of saiddifferent flows of the working fluid with different capability to absorbsolar radiation such that said capability is higher in the flow of theworking fluid adapted for the introduction in said chamber via that oneof said at least two inlets which is spaced from said window to agreater distance.
 2. A solar receiver according to claim 1, having atleast three said inlet means, wherein said absorption control meansensure that said capability to absorb solar radiation in thecorresponding different flows of the working fluid are independent ofone another.
 3. A solar receiver according to claim 2, wherein saidcapability of the working fluid, in said different flows, to absorbsolar radiation, gradually increases from a minimal capability in theflow of the working fluid adapted for the injection into said chambervia the inlet means closest to the window to a maximal capability in theflow of the working fluid adapted for the injection via the inlet meansfarthest from said window.
 4. A solar receiver according to claim 1,wherein said receiver chamber has a front wall, a rear wall and a sidewall extending therebetween, the front wall having said aperture, therear wall having said outlet means and the side wall being formed withsaid at least two inlet means.
 5. A solar receiver according to claim 1,wherein said capability of the working fluid to absorb solar radiationis defined by the concentration of solar absorbing particles in theworking fluid.
 6. A solar receiver according to claim 5, wherein saiddiffering flows of the working fluid injected into the receiver chamberhave different concentrations of said solar absorbing particles.
 7. Asolar receiver according to claim 1, wherein said chamber has across-sectional area at a region adjacent said outlet means smaller thanthat adjacent said aperture.
 8. A solar receiver according to claim 1,wherein said chamber has an elongated shaped, a front region defined bythe front wall and a front half of the side wall and a rear regiondefined by said rear wall and a rear half of the side wall, wherein saidtwo inlet means are located in the different regions of the chamber. 9.A solar receiver according to claim 1 further comprising means for saidparameters of the controlling flow rate and/or injection angles of saiddifferent flows of the working fluid.
 10. A solar receiver according toclaim 1, comprising an additional inlet for the injection therethroughof a coolant or a protecting fluid flow along said window.
 11. A solarreceiver according to claim 1, wherein each of said at least two inletmeans includes a plurality of inlet ports.
 12. A method for introducinga working fluid in a solar receiver accord to claim 1, comprisinginjecting the working fluid in different fluid flows such that, inregions farther spaced from the window, capability of the working fluidto absorb solar radiation is higher than in regions located closer tothe window.
 13. A method according to claim 12, wherein the capabilityof the working fluid to absorb solar radiation is defined byconcentration of solar absorbing particles contained therein.
 14. Amethod according to claim 12, wherein said working fluid ischaracterized by at least one parameter selected from a flow rate of theworking fluid, and an angle at which the working fluid is injected intothe chamber, and these parameters are controlled to increase thetemperature of the working fluid in the flow thereof injected fartherfrom the window.